Reactance tube modulation



M. G. CROSBY REACTANCE TUBE MODULATION Original Filed D60. 27, 1939 25 MaZ POW c 52;- 69 r- B r- C? "-5 3'2 INVE NTOR B k2 We in Me l 1 w 6: drew 6y 92 BY C5" 55 AT'ToRNEY Patented May 30, 1944 UNITED STATES- PATENT; orricr.

REACTANGE TUBE MODULATION Murray G. Crosby, Biverhead, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Original application December 27, 1939, Serial No. 311,074, now Patent No. 2,278,429, dated April 7, 1942. Divided and this application March 7,

1942, v Serial No. 433,745

7 Claims. (Cl. 1.79171.5)

figure a novel frequency modulation system is lated by the control potentials, but the module'- tion produced by undesired tube element voltage variations (not to be confused with control potentials) is cophasal so that the stability of a frequency modulated oscillator is not impaired by the reactance tubes.

A feature of my .application is a circuit arrangement wherein the modulating potentials are applied between a control electrode of one of the tubes and ground. This permits one side of the.

The present application, is a division I of oscillations. v

tube i is coupled to circuit also so that the. re-

also included.

Figure 3 is a modification of the arrangement of Figure 1; while Figures 3a and 3b are curves and graphs used to explain the operation of the reactance tube circuits.

In Figure 1, tube I, which is the reactance tube,

is of the pentode-tube type and has its anode 2 coupled by'acondenser to a tuned circuit I comprising inductance L2 and capacity C2. The

circuit 5 connects the grid 8, cathode l0, and screen grid l2 of an electron-coupled oscillator tube in regenerative circuits for the production The cathode 22 of the reactance active efllect produced intube 1 is of the oscillation generation circuit 5. t The control grid 24 of tube l *is coupled to the circuit 5 by p e shifting condenser C and the tuned reactance com- 20 prising inductance LI and condenser Ci. 'Modulating or control potentials to control the reactive eifect are supplied at 28 to the control modulation circuit to be maintained at ground potential yet provides diflerentialmodulation of the reactance tubes.

In the balanced reactance tube circuits small linear reactive changes can be obtained with change of control potentials so that the said circuits may be used to control the tuning of a tuned circuit to accomplish phase modulation of wave energy thereon. My application also disback phase shifter consists of capacitance C and closes new and improved phase and frequency modulation systems using the balanced reactance tube arrangement.

All of the modifications shown maybe used in automatic frequency control circuits operating on a heterodyning oscillator in a receiver as well as in frequency or phase modulation circuits as disclosed in application No. 136,578, filed April 13. 1937, now U. S. Patent No. 2,279,659, dated April In describing my invention in detail, reference will be made to the attached drawing wherein:

Figure 1 illustrates an improved reactance tube circuit cooperating with a source of modulating potentials and an electron-coupled oscillation enerator to modulate the frequency of operation of the latter.

Figure 2 illustrates a novel circuit arrangement comprising two reactance tubes in a balanced circuit which hasa stabilizing eiIect on the oscillation generator circuits with a novel phase shifting rid 24'.

Tube I is the reactance tube which produces the reactive effect in its plate circuit. The feedtuned circuit Ll-Cl shunted by resistance R. That is, high frequency voltage from the circuits of tube 20 is supplied to the anode 2 of tube I and through the phase shifter circuit including C, Li, Cl, and R to the grid of tube I. This phase shifter is of the constant-current typein which the reactance of the capacitance C is made large compared to the impedance of the tuned circuit Li-Cl. The capacitive reactance, therefore, determines the phase of the current in the said circuit-so thatv the potential drop across the resistive tuned circuit Ll'-C|, which is resonated at the frequency for which the reactive effect is being obtained, takes the phase of the current and leads the plate voltage by practically Since the current pulses which reach the anode 2 lead the voltage on the said anode by about 90, a reactive effect is produced in the tube and thisreactive eifect, which is in this case capacitive, is eflective in the circuit L2--G2 which controls the frequency of operation of the oscillation generator. The degree of reactive effeet is controlled by the bias potentials on the grid 24. If the. grid is made mor positive the reactive effect increases. If the gridbecomes more negative the reactive efiect is decreased.

circuit and novel modulation circuit. In thisoli.

-If the frequency range over which the reac-, tance is to operate is large compared to the selectivity of circuit Ll--Cl, the damping effect I fect provided by the tube i v to be tuned from Figure 1 in that a phase, the voltage on th capacitance C of B may be increased by making R lower. If a smaller range of frequency control, but a high sensitivity to control potential is desired, resistor It may be removed completely. Reducing the value of C increases the reactive eflect, but unless the reactance or C is made large compared to the impedance of the tuned circuit, a resistive iiect is introduced in the variable reactance ef- Direct-current voltage is fed to the anode 2 of reactance tube I through radio-frequency choke RFC. This voltag may I by means of a direct connection to the oscillator tuned circuit L2-C2 if the oscillator tuned circuit is of the type which has the plate voltage on it. Capacitance}! blocksthe reactance tube plate voltage from the tuned circuit L2-C2 of the oscillator tube 20. The oscillator tube is of the electron-coupled type which supplies its output from tuned circuit L3-C3 electronically coupled to the generating circuits and electrodes. Plate voltage is supplied by. lead 23 to the anode of tu 20. Voltage' for the. screen grid l2 of tube 20 is s pplied from the plate source by means of potentiometer resistances 21 and 28. The oscillations generated and controlled as to frequency in accordance with controlof the grid 24 of tube I is supplied through coupling condenser 150 any utilization means. The circuit if controlled at 24-26 by modulating potentials produces frequency modulated waves. If controlled by AFC potentials oscillations of substantially constant frequency are produced.

In the circuit of Figure l, the feed-back ca-' pacitance C ,may be eliminated and inductive coupling effected between inductances Li and L2. This would make the circuit a single-tube equivalso be supplied to oppose that fed to circuit 86 by C. The opposition is caused by the back by C is at the opposite end of coil 58 which ,has the opposite polarity. This opp'oslngfeedback would completely neutralize the reactive effect if C and C were made equal. However, this equality is intentionally avoided by making C larger than C so that the predominate feedback is dueto C. This causes age appearing on grid 54 to be 180 out of phase with that fed to grid 24 since that voltage is predominantly fed from C through coil 58.

"Since the tuned circuit consisting of 58 and 53 is tuned to the frequency of the energy which isto be modulated, this circuit is resistive and, therefore, acts as though pure resistances were connected from grids 24 and 54 to ground. Hence capacity C in conjunction with this resistive eflect forms the phase shifter which produces 'a 90 grid 24. Capacity 0 is adreactance compared to the so that the current flowing displaced volta e on justed to be of high resistive effect of 58 is determined by the capacity and is, therefore,

leading the applied voltage from the anode. The drop across the resistive eflect will, therefore, be leading like the current in a condenser and when this voltage is. amplified through the tube, the current flowing in the plate circuit of the tube is also leading and the reactive effect is the same as a condenser. The current flowing through the. condenser C has thus been amplified by the tube so that the plate circuit of the tube presents a capacitive eflect which is variable by th amount that the amplification of the tube is variable.

The voltage appearing on grid 54.01 tube 52 in 180 out of phase with that appearing on the grid alent of the circuit of Figure 3 of my United States Patent No. 2,250,095.

The circuit of Figure 2 shows a balanced type of 'reactanc'e tube circuit which I have found to, be very suitable for the production of oscillations of constant frequency due to advantages obtained by the balance. In my United States Patent No. 2,250,095, in Figure 3, I show a circuit employing this principle. In the circuitof said application "ing in the plate of tube 52 is inductive and has an effect on tuning which is opposite to that of I employ inductive coupling between the circuit circuits of the reactance tube i in the same manner as in Figure 1. However, Figure -2. aura-2 second tube 52 is provided which gets on its grid 54 a feed-back voltage 180 out of phase with the voltage on the grid 24 of tube I. That is, in-the niodiflcationof Figure 2, the grid 24 derives a voltage through C of-a particular phasewhile the grid 54- of tube I52 gets by way of the tuned circuit comprising inductance 56 and condenser 58a voltage substantially 180 out of phase relative to the said voltage on grid 24. That is, if we assume zero phase of the voltages on anode 2 the voltage impressed by Con grid 24 is shifted 90' while the voltage im-; pressed. on-the grid 54 is shifted 180 more or 210 relative to the zero phase voltages on theanode24 of tube i.' Furthermore, if we look at the voltages on the a ode 53 as being of zero the reactive .C is employed between the plate 2 and the grid tube I to produce a compensating capacitive regrid 56 is displaced by of tube i. This makes the voltage on 54 lagging with respect to the anode voltage. The tube then amplifies a current suclias would flow through an inductance sothat the reactive eflect appeartube I. When these two opposing tuning eflects produced by the two tubes are properly balanced,

the advantage of stability as well as improved range and linearity of reactance variation are obtained. Thus, if variations in the direct-cub.

rent supply potential to an electrode in the tubes causes variations in the current impulses. the resulting reactive changes-are opposed and the total reactive eflect is balanced. -This causes the actance which neutralizes the inductive rea'ctan'ce of the tube 52 aslong as the amplifications oi the two tubesare balanced. As a consequence,

in the absence of modulating potentials and-inthe presence of variations in power supplypotentials on tube electrodes. my tube reactance circuit tends to'stabilize theoscillation generator and compensate for said potential variations so that oscillations of substantially constant frequency are generated. Obviously, when the advantages gained by the use of anelectron-coupled oscillator are combined with the advantages gained by the use of my self-compensating reactive tube arrangement, oscillations. of substantially constant frequency are obtained irrespective of varlations in tube electrode voltages.

Diil'crential modulation'of tube' lectrode voltages, such as the. screens, suppressor grids, control grids, etc... however, causes an increase in the current impulses to. anode 2 atthe same time i that it causes a decrease inthe current impulse to the anode as and vice versa; As a result, they fact that the voltage fedthe resulting volt-' of 58 to grid ll produce an increase oi the shunt capacitance at the same time that they produce an increase in the shunt inductancemr vice versa.- n the other" hand, any cophasal modulation of the element voltages produces an effect in one tube which is compensated for by the other tube so that such undesirable variations are eliminated.

Th inductance 56 and condenser 58 are tuned to substantially the mean frequency at which the oscillator 20 is to operate. The reactive eflect appears inthe common plate circuit of the two 4 tubes which may, as shown, be coupled by condenser 3 to the oscillator tuned circuit L2-C2 forv frequency stabilization and/or irequency modulation. The reactance effects obtained by my invention may be applied to the tuned circuit of an amplifier and maintained under complete control of the modulating potentials. As a consequence, this arrangement is well adapted to use for phase modulation.

Variable capacitance C is the feed-back coupling condenser which corresponds to condenser C in Figure 1. This capacitance may be composed of the internal grid-to-plate capacitance of the tube" together with added external capaci-- tance, if necessary, or, as I have found, may consist of the unbalance between the grid-to-plate capacitances of the two tubes 2 and 52 and their wiring. Complete control of this capacitance is obtained when C is larger than the condenser C which may be connected between the grid 54 and plate 52 of the second tube so that the two may be relatively adjusted for complete neutralization or any degree of oil-neutralization desired. The capacitance C usually is composed of the tube capacity only and for that reason is shown by dotted lines.

Figure 2 shows how the balanced circuit may be arranged so as to be controlled by a control potential supplied at 26 with one side at low or 0 ground potential while at the same time retaining the advantage of the balanced reactance tube arrangement and differential modulation. In this 1 arrangement grid control is used and the control potential circuits are connected as shown to the grid 54 01' tube 52. The reactive eifect will be completely balanced out when the control potential is zero. Whenepositive control potential is applied, the grid 54 oitube52 is made more positive. This causes tube 52 to draw more cathode current, and, therefore, places both cathodes at a higher positive potential due to the potential drop in cathode resistance I0 shuntedby bypass condenser I2. This higher positive cathode potential is equivalent to a higher negative potential on the grid 24 of tube i. Thus, the required differential control of the gain of the tubes I and 52 by the control potentials is obtained.

The common cathode resistor I0 causes the grid potential of the-one tube to vary in the opposite direction to that of the other. However, this common cathode resistor III is not absolutely necessary because the circuit is difl'erentially unbalanced -by varying an element voltage of one tube, say 52, without varying that of theother tube, say I. The common cathode resistor III tends to increase the sensitivity to control potentials and increases the range of reactance variation obtainable.

The wave generator dicated by a block diagram at 20"may be as lustrated in Figure 1 or of any typ known in the art. The reactive eflect provides by the balanced arrangement comprising. tubes I and 52 stabilizes the operation of the generator which may also be controlled by potentials at 26, A common direct-current source or sources which vary in potential in synchronism rangement. 5 Figure 3 is a circuitoi the balanced type which uses phase shifters which are untuned. The phase shifter for tube I consists of resistance RI and capacitance C2. RI is made large compared to the reactance of'C2 so that the vector relations mxoithe circuit are as shown in Figure 3a. In

Figure 30, E is the applied voltage which is supplied fromthe common plate circuits of tubes I and 52 and I is the current flowing in the phase shifter circuit RI, C2. RI is the voltage drop across resistor RI and Kai is the voltage drop across capacitance C2. The angle vbetween the applied voltage E and the drop across the capacitance C2, which is fed to the grid of tube I, is seen to be substantially 90 and, more nearly approaches 90 as the ratio between RI and C2 is increased.

The phase shifter for tube 52 consists otcapacitanc C3 and resistor R2. In this case, the reactance of C3 is made large compared to resistance R2 so that the current is substantially reactive. The vector diagram for the phase shifter R2, C3 is shown in Figure 3b. It can be seen that this ,type of phase shifter-produces a circuit of Figure 3 by means of the push-pull .feed from transformer 90 to the.suppressor grids 9i and 92. Capacitances BPC are radio-frequency by-pass condensers and 94 is a blocking condenser. Resistor 96 furnishes a direct-current return for the grid 24 of tube I.

The reactive eflect supplied to generator 20' stabilizes operation of the same which then may be controlled by potentials from 26. Direct-current voltages which vary similarly are used here as in the prior modifications.

In the circuit 01 Figure 3, capacitances C2 and C3 may be replaced by inductances or the same reactance and the same balance action will be obtained. In all 01' these balanced circuits there are two methods by which the reactive effect 0 one tube may bebalanced against that 01' th other so that element voltage supply variations will be neutralized. One of these methods is to balance the amount,of voltage fed back to each tube. This might be done by varying the relative sizes of the two halves vo1 the inductance 5B in Figure 2, or by varying the values of RI, C2, and R2, C3, in Figure 3. In any of these cases the balance would be made by increasing the voltage fed back to the tube which did not produce a large enough reactive effect. The second method of balance is by means of the screen-grid balancing potentiometer. In the case of Figure 3, it has been round convenient in practice to have both of these types of balance available and to do so C3 was made variable and a screen-grid balancing potentiometer 68 -is included also.

What is claimed is:

supplies the generator and reactance tube ar- 1. In combination with a circuit wherein alterof said devices in shunt to at least part or said circuit, reactance means for impressing phase displaced currents from said circuit on two electrodes in each of said devices to produce in said devices reactive effects, a biasing impedance in a common connection between the cathodes of said devices and a point of substantially fixed. al-

nection between the'control electrode of one device and said point potential connected between said'point and the control electrode of the other between the emission electrodes of said devices and said source toshunt said source by the ii ternalimpedances between said receivingelectrodes and emission electrodes ofthe devices, and means for impressing opposed voltages on said control electrodes which are s also displaced in phase substantially 90 relative to the voltages on 'said'receiving electrodes including, aparallel cirof said devices only, the connections including said impedance b'eing'such that variations of the control pctential'on said one control electrode produces variations in the potential drop in said impedance which changesthe bias on the other grid in an opposite sense to differentially control the conductances of the devicesto thereby control the reactive efiects produced.

2. In combination with a circuit wherein alternating current fiows and across which a controllable reactive effect is to be produced, a pair of electron discharge devices each having an anode, a cathode, and a control electrode, a circuit parallel tuned to the frequency of said alternating current connected between the control electrodes of said devices, a; connection of low impedance to current of the frequency of said alternating current between a point on said'last circuit and the cathodes oi said devices; connections coupling the internal impedances between the anode and cathode of each of said'devices in shunt to a part at least of said first circuit, a reactance coupling the anode of one of said devices to the control electrode of said one device, an impedance connected between the cathodes of said devices and the control electrode of said one of said devices, and a source of control potentials in series with said impedance between the control electrode of the other'of said devices only and the cathodes of cuit'parallel tuned to the frequency or said g ternating current potential, a direct current con-,1

source of alternating current coupled between the control electrodes oi said devices, a coupling besaid devices.

5. In a signalling system, a circuit wherein al ternating current flows and. across which a controllable reactive eilect is to be provided, an electron discharge device having anelectron receiving electrode, an electron emission electrode, and an electron flow contro1 electrode, means coupling the impedance between the electron receiving electrode and electron emission electrode of said device in shunt to the reactance in said circuit wherein said alternating current flows, said coupling setting up on said receiving electrode alternating voltage of a first phase, an inductance and a condenser in parallel tuned to'the frequency of said alternating current, a reactance coupling the electron receiving electrode oi. said said devices, the connections including said imv pedance being such that variations of the ,control potential on said one control electrode produces variations in the potential drop in said impedance which changes the bias on the other control electrode inan oppositesense.

3. In a wave length modulation system, a circuit wherein oscillation energy, the wave length or which is to be modulated flows, a pair of electron discharge devices each having an anode, a

cathode and a control grid, means couplingthe internal impedance between the anode and cathode'of each of said devices in shunt to the reactance insaid first mentioned circuit, an inductance and a condenser in parallel'tuned to the frequency 'of' said oscillatory energy, a con; denser coupling the anode of one device to the control grid of said one device conductors COB-g ,necting'said parallel inductance and condenser between the control grids of said devices, a coupling between a point on said parallel circuit and the cathodes of said devices, an impedance connectingthe cathodes ofsaid devices to ground, and a source of modulating potentials connected nating voltage of a first phase, an inductance and between"the grid of one device only and ground.-

i 4. In a controllable reactance, a-pair of electron discharge devices each having an' electron receivingelectrode, an electron emission electrode and a control electrode, a sourceot alternating, current, means tying the'receivi'ng electrodes together and connecting thesame-to said source to set up on said receiving electrodes al;

ternating voltages of a ilrstphasa'connections device to the electron flow control electrode oi! said device, conductor coupling said parallelinductance and condenser between the electron flow control electrode and electron emissioh electrode of'sa'id device, said coupling reactance'and inductance in parallel being arranged to set up on said electron fiow control electrode,'an alternating voltage which is substantially inphase quadrature with respect to the alternating voltage on said electron receiving electrode and connections for controlling the gain of said device in ac cordance with control potentials to thereby control the reactive eilect produced in said device and added to the nuisance in said first mentioned circuit. 7

-6. In a signalling system, a circuit wherein alternating current flows and across which a controllable reactive effect is to be provided, an electron discharge device having an electron receiving electrode, an electron emission electrode, and an electron fiow control electrode, means coupling the impedance between the electron receiving electrode and the electron emission electrode. of

said device in shunt to the reactance in said circuit wherein alternating currentjfiows, said .coupling setting up on said receiving electrode altera condenserin parallel tuned to the frequency of said alternating current, a 'reactance c'ouplingtlie electron receiving electrodeof said device to the electron flow control electrode of said device, conductors coupling said parallel inductance and condenser between the electron flow control electrode and electron emission electrode of said depfiow' control electrode, an alternating voltage which is substantially. in phase quadrature with. respect to the alternating voltage on said elec-- vice, said coupling reactance andinductance in parallel being arranged to set up on-said electron tron receiving electrode a resistance in, shuntto said parallel inductance and'condenserand conassaeii nections for modulating the gain of said device in accordance with control potentials to thereby control the reactive eirect produced in said device and added to said first named circuit.

'L'In a wave length modulation ystem, a circuit wherein oscillatory current the wave length of which is to be modulated flows, 9. pair of electron discharge devices each having an electron receiving electrode. an electron emission electrode, and an electron flow control electrode, connections coupling the internal impedance between the electron receiving electrode and electron emission electrode of each of said devices in shunt to a part at least of the reactance in said 'iirst mentioned circuit, on inductance and a condenser in parallel tuned to the frequency of said oscillatory current, a reactance coupllm the electron receiving electrode of one device to the elec- Ill tron flow control electrode of said one device. conductors coupling said parallel inductance and condenser between the electron flow control electrodes of said devices. a coupling between said parallel inductance and condenser andthe electron emission electrodes of said devices, said coupling reactance and parallel tuned circuit serving to set up on the electron flow control electrodes of said devices opposed voltages of the frequency of said oscillatory current which are substantially in phase quadrature relative to the oscillatory voltages on the electron receiving electrodes of said devices, a source of modulating potentials and connections between said source of modulatins potentials and one of said device only for modulating the gain thereof in accordance with signals.

. I MURRAY G. CROSBY. 

